Photochromic glasses containing silver and one or more halogens in their compositions are well known. Normally, such glasses are clear and uncolored as produced. However, when exposed to ultraviolet radiation, a photochromic glass becomes light absorbing over a broad range of wavelengths in the visible portion of the spectrum. The absorption is reversible, that is, the glass returns to the clear state when the exciting radiation is removed.
Various photochromic materials are known. However, the commercially viable material at the present time is a glass containing one or more silver halide crystal phases selected from AgCl, AgBr and AgI. As melted, silver and halogen are dissolved in a glass. Upon a controlled thermal treatment, as amply described in the patent literature, a silver halide crystal phase is precipitated in the glass. The crystals constituting this phase are subject to photolysis, thereby giving rise to photochromism, that is, reversible coloring behavior. As used herein, the terms "halide" and "halogen" refer to chlorine, bromine and iodine, fluorine not being effective to impart photochromic behavior.
If desired, it is possible to impart permanent coloration to photochromic glasses with conventional glass colorants. These include such known colorants as NiO, CoO, Cr.sub.2 O.sub.3 and MnO which may be incorporated in a glass batch in usual manner.
In the recent past, another method of inducing permanent coloration in a photochromic glass was discovered. This method involved a high temperature thermal treatment in a reducing atmosphere, such as hydrogen. The thermal treatment can bring about a permanent, partial or complete reduction of silver halide to silver. A wide range of colors can be produced that depends on the temperature of the reduction treatment. This method is described in detail in U.S. Pat. No. 4,240,836 (Borrelli et al.).
Although the exact mechanism of this induced coloration is not well understood, it is proposed that the color depends on the extent of reduction of the silver halide to silver. The reason for this hypothesis is threefold. First, the color produced by silver particles in glass is yellow. This results from a sinqle absorption band at 400 nm, where it would be predicted to appear from simple scattering theory. The appearance of absorption bands, which are considerably shifted toward the red, indicate the presence of something other than simply reduced silver in glass. Second, the colors produced by the reduction depend on the temperature of the treatment alone. Chemical reduction leading to a greater amount of silver would not be expected to change the spectral position of the absorption band as is observed. Third, the color variation can only be obtained if the hydrogen treatment is carried out below the melting temperature of the silver halide phase in the glass. This suggests the presence and critical role of the silver halide phase.
One should recognize that the color produced by the hydrogen reduction resides in a surface layer corresponding to the depth of diffusion of hydrogen. It can be shown that, in situations where the diffusion is accompanied by a fast chemical reaction, the layer thickness is proportional to the square root of time of the reduction treatment. The remaining portion of the glass is the uncolored photochromic glass, and exhibits the usual photochromic behavior.
It is a primary purpose of the present invention to provide an alternative means of achieving permanent coloration effects in a glass having a silver halide crystal phase. The coloration effects are similar to those previously achieved by hydrogen atmospheric reduction. Both methods involve a thermal reduction step, but the presently proposed method has a number of advantages.
A primary advantage is that the need for treatment in a hydrogen-containing atmosphere is avoided. Another advantage is that the present method is effective in the same temperature range as that found effective to induce photochromic behavior in potentially photochromic qlasses. This avoids the need for a separate thermal treatment. Another advantage is that the present method is effective in certain glasses that contain a silver halide phase, but do not exhibit photochromic behavior. A further advantage is that color is developed throughout the glass; not just in a surface layer.